Potentiometric oxygen sensors with an internal reference electrode, which uses the equilibrium pO2 of the binary mixture of Ni/NiO as the reference, are demonstrated. The cells employ Pt or composite ceramics as the sensing electrode. The cells are fabricated by a flexible and potentially low cost method and show high accuracy. The errors in cell voltage are less than 4 mV above 359°C. The cells exhibit good stability (>5100 hrs), extended working temperature range, good tolerance to thermal and pO2 cycling, fast response and easy recoverability when Ni is depleted. Both cell fabrication and performance are highly reproducible. The composite ceramics, based on strontium doped manganite and yttria doped zirconia, are proven superior over Pt to serve as the electrode material.